it is supported. its rather your choice whether you prefer it over nextclouds built in note app, or things like carnet (another note taking app with nextcloud integration)

how to set it up?checking their website at https://joplin.cozic.net/ you can read(already changed the url to reflect disroot):

On the desktop application or mobile application, go to the config screen and select Nextcloud as the synchronisation target. Then input the WebDAV URL (to get it, click on Settings in the bottom left corner of the page, in Nextcloud), this is normally https://cloud.disroot.org/remote.php/webdav/Joplin

(make sure to create the "Joplin" directory in Nextcloud), and set the username and password. If it does not work, please see this explanation for more details.

of course you can use any folder you want instead of joplin. so for example if i want joplin notes to be part of my document folder the path should be:

In case of carnet it is using its own file encoding so probably not possible to use other text editors. With Joplin although it uses markdown I read in number of places that it tends to break the layout when opened in different then joplin editor. But yeah, if you are on lookout, check all the options and choose what fits your needs the most.

Yes. I do like the nextlcoud notes as I use them for just that, notes. No need for fancy formatting and all. Though if you require more from note taking app joplin is a good choice, but you should use joplin editor both on mobile and desktop to not break the formatting.